Method and system for subsea processing of multiphase well effluents

ABSTRACT

A method and system are disclosed for subsea processing multiphase well effluents comprising natural gas and liquid from a subsea hydrocarbon containing formation, which system ( 1 ) comprises: —a fluid separation vessel ( 6 ) which is connected to a downstream end of a multiphase well effluent transportation conduit ( 7 ); —a liquid level transmitter assembly ( 21,22 ) for monitoring the gas liquid interface ( 23 ) in the fluid separation vessel ( 6 ); —a liquid enriched fluid transportation flowline ( 14 ) connected at or near the bottom ( 13 ) of the fluid separation vessel ( 6 ) and a gas enriched fluid transportation flowline ( 12 ) connected at or near a top ( 11 ) of the fluid separation vessel ( 6 ); —a pump ( 17 ) connected to an electric motor ( 18 ); and —a fast acting variable speed drive system, which is coupled to the liquid level controller which adjusts the pump and motor speed setpoint within 2 seconds to maintain the liquid level in the vessel at a predetermined setpoint.

BACKGROUND OF THE INVENTION

The invention relates to a method and system for subsea processing ofmultiphase well effluents.

U.S. Pat. No. 7,210,530 discloses a fluid delivery system comprising avertical liquid/vapor separator and riser assembly that comprises amulti-phase separator inlet, a vapor outlet and riser, and a liquidoutlet port connected to a hydraulically-driven centrifugal pump. Bycontrolling the operational speed of the pump, the level of theseparated liquid within the separator can be controlled without the needfor control valves. The vertical separator is capable of low pressureoperation and large variations in controlled liquid levels within theseparator, allowing the relatively slow reaction time pump to controlthe liquid level from low pressure wells penetrating low pressurereservoirs.

There is a need in the art for one or more of the following:

A hydraulic pump with a response time of less than about twenty seconds,or less than about sixty seconds;

A hydraulic pump combined with a differential pressure transducer tocontrol the flowrate of the liquid flowing through the liquid flowline;and/or

A subsea processing system without the need for a low level shutdownsystem to protect the pump from mechanical damage if the separator isdrained.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method of subseaprocessing multiphase well effluents produced from an underwaterhydrocarbon containing formation, the method comprising:

-   -   arranging a fluid separation vessel at or near the seabed;    -   inducing the multiphase well effluents to flow from the        formation through a well and an underwater multiphase well        effluent transportation conduit to the fluid separation vessel;    -   separating the mixture in the fluid separation vessel into gas        and liquid enriched fluid fractions;    -   transferring the liquid and gas enriched fluid fraction to        surface crude oil gas processing facilities via separate        flowlines;    -   boosting the pressure in the flowline for transferring the        liquid enriched fluid fraction by a pump of which the pump speed        is adjusted in response to variation of a gas liquid interface        level in the separation vessel;    -   wherein the pump is driven by an electric motor of which the        speed is controlled by a fast acting variable speed drive        system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a subsea processing system according tothe invention.

DETAILED DESCRIPTION OF THE INVENTION

A liquid level transmitter measures the liquid level in the separationvessel and transmits the level signal to a controller, which adjusts thepump speed setpoint in a cascade arrangement to maintain the liquidlevel at a predetermined setpoint. The level controller is tuned toprovide extremely fast action for handling level disturbances caused bychanges in the liquid feed rate, such that the pump speed setpoint isalmost instantly adjusted to respond to changes in liquid level. Howeverthe level controller also provides stable pump speed when the liquidfeed rate is stable.

The fast acting variable speed drive receives its speed setpoint fromthe level controller and increases or decreases the pump speed within 2seconds, being limited only by the physical capabilities of theparticular variable speed drive. This fast acting level controllerenables the use of compact separation systems as the liquid interfacelevel can now be maintained within a small boundary.

It is also preferred that the pump is a gas tolerant or multiphase pump,to ensure occasional separator drainage or gas surge can be toleratedwhen severe liquid feed disturbances occur which may require a fasterpump speed response than is physically possible with the variable speeddrive.

It is furthermore preferred that:

-   -   the flowline for transferring the gas enriched fluid fraction is        connected at or near the top of the separation vessel and the        flowline for transferring the liquid enriched fluid fraction is        connected at or near the bottom of the gravity separation        vessel, that the flowline for transferring the gas enriched        fluid fraction is equipped with a liquid overflow valve, which        is closed if liquid enters the valve,    -   the separation vessel and liquid valve overflow valve are        arranged in a separator module which is retrievably mounted on a        pump station base at the seabed and    -   the pump and the electric pump motor are mounted in a pump        module, which is retrievably mounted on the pump station base.

Furthermore, an inlet of a fluid recirculation conduit may be connectedto the liquid enriched fluid transportation conduit at a locationdownstream of the pump and an outlet of the fluid recirculation conduitis connected to the multiphase well effluent transportation conduit,which conduit is equipped with a valve, which is opened during start upof the pump and which is subsequently closed.

In accordance with the invention there is furthermore provided a systemfor subsea processing multiphase well effluents comprising natural gasand liquid from an underwater hydrocarbon containing formation, thesystem comprising:

-   -   a fluid separation vessel which is connected to a downstream end        of a multiphase well effluent transportation conduit;    -   a liquid level transmitter assembly for monitoring the gas        liquid interface in the fluid separation vessel;    -   a liquid level controller which receives the signal from the        level transmitter and generates a pump speed setpoint for        controlling the liquid level to a predetermined setpoint;    -   a liquid enriched fluid transportation flowline connected at or        near the bottom of the fluid separation vessel and a gas        enriched fluid transportation flowline connected at or near a        top of the fluid separation vessel;    -   a pump connected to an electric motor;    -   a fast acting variable speed drive system, which is coupled to        the liquid level controller which control the pump and motor        speed as fast as physically possible, within 2 seconds, to        achieve the pump speed setpoint from the level controller.

These and other features, embodiments and advantages of the method andsystem according to the invention are described in the accompanyingclaims, abstract and the following detailed description of a preferredembodiment disclosed in the accompanying drawing in which referencenumerals are used which refer to corresponding reference numerals thatare shown in the drawings.

FIG. 1:

FIG. 1 shows a subsea processing system 1 according to the inventioncomprising a separation module 2 and a pump module 3 that areretrievably mounted on a base 4, which is mounted on the seabed 5.

The separation module 2 comprises a gas-liquid separation vessel 6,which is connected to a multiphase well effluent flowline 7, which isconnected via a manifold 8 to a pair of subsea gas wells 9 and 10 whichproduce a multiphase flow mixture of natural gas, water, condensates andoptionally crude oil from an underwater gas reservoir (not shown). Thegas-liquid separation vessel 6 is provided with an upper outlet 11 fordischarging a gas enriched fluid fraction into a gas transportationconduit 12 and with a lower outlet 13 for discharging a liquid enrichedfluid fraction into a liquid transportation conduit 14. The gas andliquid transportation conduits 12 and 14 comprise vertical risersections 12A, 14A, which are suspended below a floating platform 15,which floats at the water surface 16.

In order to enhance the liquid production and reduce the backpressure ofthe hydrostatic liquid column in the liquid filled riser section 14A onthe separation vessel 6 the pump module 3 comprises a gas tolerantliquid pump 17, which is driven by an electric motor 18 that is poweredvia an electrical cable 19 by an electric power source 20 at theplatform 15.

The electric motor 18 is equipped with a fast acting variable speeddrive system, which is coupled, as indicated by dashed line 24, to anassembly of two liquid level transmitters 21 and 22 and a levelcontroller which monitor the liquid level 23 in the separation vessel 6.The level controller provides the speed setpoint to the fast actingvariable speed drive system. The level controller will increase thespeed of the motor and pump assembly 17,18 within 1 to 2 seconds if theliquid level increases above its setpoint 21 and the level controllerwill decrease the speed of the motor and pump assembly 17,18 within 1 to2 seconds if the liquid level 23 drops below the predetermined liquidlevel controller setpoint 21.

In order to inhibit liquid to flow into the gas transportation conduit12 in case of a liquid surge a liquid overflow valve 25 is arranged inthe gas transportation conduit 12 in the vicinity of the upper gasoutlet 11, which valve automatically closes itself if a liquid surgeenters the valve.

During start up of the pump 17 at least a fraction of the liquiddischarged from the lower liquid outlet 13 is circulated back to themultiphase well effluent flowline 7 through a liquid recycling conduit26, which is equipped with a valve 27, which is opened during the pumpstart up phase and which is subsequently gradually closed. The valve 27may comprise a conventional subsea choke and an on-off valve, which doesnot need to be fast acting as failure to control the recycle flow ratewill not have a detrimental effect on the multiphase pump 17.

Thus an advantage of the subsea processing system according to theinvention is that the valves 25 and 27 may be simple on off valves,which do not require a complex dynamic control of the valve openings andthat the electric motor 18 is the only subsea component which isdynamically controlled by a fast acting variable speed drive systemwhich receives its speed setpoint from a level controller.

The modular construction of the system 1 allows replacement, inspectionand/or repair of the retrievable pump and separator modules 3 and 2 bydisconnecting the stab-in pipe joints 30 and the subsea power cableconnector 31 and then lifting the pump and/or separator module 3,2 to avessel at the water surface 16 for replacement inspection and/or repair.

It will be understood that the processing system 1 according to theinvention may be installed at the bottom 5 of an ocean, sea, lake and/orriver and that the term “subsea” as used in this specification and theaccompanying claims should be interpreted extensively so that itembraces all these options.

1. A method of subsea processing multiphase well effluents produced froman underwater hydrocarbon containing formation, the method comprising:arranging a fluid separation vessel at or near the seabed; inducing themultiphase well effluents to flow from the formation through a well andan underwater multiphase well effluent transportation conduit to thefluid separation vessel; separating the mixture in the fluid separationvessel into gas and liquid enriched fluid fractions; transferring theliquid and gas enriched fluid fraction to surface crude oil gasprocessing facilities via separate flowlines; boosting the pressure inthe flowline for transferring the liquid enriched fluid fraction by apump of which the pump speed is adjusted in response to variation of agas liquid interface level in the separation vessel; wherein the pump isdriven by an electric motor of which the speed is controlled by a fastacting variable speed drive system which is coupled to a liquid levelcontroller, which monitors the liquid level in the separation vessel andwhich increases the pump speed within 2 seconds if the liquid levelrises above a predetermined liquid level setpoint and decreases the pumpspeed within 2 seconds if the liquid level drops below the predeterminedliquid level setpoint.
 2. The method of claim 1, wherein the fast actingvariable speed drive increases or decreases the pump speed in a cascadearrangement within 1 to 2 seconds if the liquid level is below thepredetermined minimum level or above the predetermined maximum level. 3.The method of claim 1, wherein the pump is a gas tolerant multiphasepump.
 4. The method of claim 3, wherein the pump is a positivedisplacement pump.
 5. The method of claim 4, wherein the pump is a twinscrew pump.
 6. The method of claim 1, wherein the fluid separationvessel is a gravity separation vessel and the flowline for transferringthe gas enriched fluid fraction is connected at or near the top of thegravity separation vessel and the flowline for transferring the liquidenriched fluid fraction is connected at or near the bottom of thegravity separation vessel.
 7. The method of claim 1, wherein the fluidseparation vessel is a compact, for example cyclonic, separation vesseland the flowline for transferring the gas enriched fluid fraction isconnected at or near the top of the gravity separation vessel and theflowline for transferring the liquid enriched fluid fraction isconnected at or near the bottom of the gravity separation vessel.
 8. Themethod of claim 6, wherein the flowline for transferring the gasenriched fluid fraction is equipped with a liquid overflow valve, whichis closed if liquid enters the valve.
 9. The method of claim 8, whereinthe separation vessel and liquid valve overflow valve are arranged in aseparator module which is retrievably mounted on a pump station base atthe seabed.
 10. The method of claim 9, wherein the pump and the electricpump motor are mounted in a pump module, which is retrievably mounted onthe pump station base.
 11. The method of claim 1, wherein an inlet of afluid recirculation conduit is connected to the liquid enriched fluidtransportation conduit at a location downstream of the pump and anoutlet of the fluid recirculation conduit is connected to the multiphasewell effluent transportation conduit, which conduit is equipped with avalve, which is opened during start up of the pump and which issubsequently closed.
 12. The method of claim 11, wherein the liquidoverflow and pump recycle valves are slow acting valves, which isconfigured to be moved between a fully opened and a fully closedposition over a time interval of typically around 20 seconds.
 13. Asystem for subsea processing multiphase well effluents comprisingnatural gas and liquid from an underwater hydrocarbon containingformation, the system comprising: a fluid separation vessel which isconnected to a downstream end of a multiphase well effluenttransportation conduit; a liquid level transmitter assembly formonitoring the gas liquid interface in the fluid separation vessel; aliquid enriched fluid transportation flowline connected at or near thebottom of the fluid separation vessel and a gas enriched fluidtransportation flowline connected at or near a top of the fluidseparation vessel; a pump connected to an electric motor; a fast actingvariable speed drive system, which is coupled to the liquid levelcontroller which provides the pump and motor speed setpoint in a cascadearrangement to control the level to a predetermined setpoint, and whichis configured to bring the pump and motor speed to the designated speedsetpoint within 2 seconds.
 14. The system of claim 13, wherein the fastacting variable speed drive system is configured to increase anddecrease the pump and motor speed in a cascade arrangement within 1 to 2seconds, to achieve the speed setpoint provided by the level controller.